The role of macrophages in the activation of T-lymphocytes induced by mitogens is under investigation. T-lymphocytes do not proliferate at all in response to PHA or Con A unless macrophages are present. Macrophages function in two ways in this response. They bind the mitogen to their surface and "present" it to the T-cell or they release a soluble factor, termed lymphocyte activating factor (LAF), which will enable T-cells that have themselves bound mitogen to proliferate.